With an increase in electrical power of spacecraft in recent years, a phenomenon of electrical charge and discharge on a solar cell array of the spacecraft caused by space plasma has become a big problem. Since electrical power needed for realizing various functions of the spacecraft is obtained through solar-cell power generation, damage to the solar cell array caused by electrical discharge may result in total loss of the spacecraft. That is, avoiding electrical discharge at the solar cell array directly leads to an increase in the service life of the spacecraft.
FIG. 25 shows a schematic view of a spacecraft (satellite), and a detailed sectional view of a portion of a solar cell array thereof. The front surface of a panel to which solar cells are attached is covered almost entirely with an insulating tape. No insulating tape is bonded or welded to the back surface of the panel. The solar cell array is composed of a large number of solar cells (basic units) which are connected in series via inter connectors and each of which has electrodes attached to opposite sides thereof. The solar cells, whose front surfaces are covered with cover glasses (insulating material), are attached onto the structure of the spacecraft via an insulating tape. A spacecraft located in a geostationary orbit environment may encounter an unexpected substorm (magnetospheric substorm) generated due to solar activity in some times, and such a substorm is a main cause of a malfunction of the spacecraft. Electrons of high energy impinge on the spacecraft, and the spacecraft has a negative potential of a few kV or greater. As a result, the insulator on the surface of the spacecraft has a potential different from that of the spacecraft. An strong electric field is produced at a location (triple junction) on the surface of the spacecraft at which a vacuum, an insulator, and a conductor (which has the same potential as the spacecraft) meet, and an electrical discharge occurs. Many triple junctions are present on the solar cell array due to its structure, and an electrical discharge causes short-circuiting of a circuit, resulting in stoppage of supply of electrical power.
Non-Patent Document 1 discloses a technique of exposing a triple junction (location where an insulator and a conductor meet) to a space, to thereby emit electrons. However, Non-Patent Document 1 does not disclose concrete means.
Non-Patent Document 2 discloses a technique for preventing a satellite from acquiring a charge. Specifically, a charged state is detected by use of a charge sensor, and a high-density plasma is jetted to thereby increase the potential of the satellite, and eliminate potential differences between portions of the satellite. However, the employed control is basically an active control utilizing a sensor and a plasma injector, and therefore, a sensor, a power supply, a gas, etc. must be mounted on the satellite.
Patent Document 1 discloses a technique for controlling the potential of a satellite by use of a photocathode electrically connected to the satellite, and a light source for exciting the photocathode. However, this technique has a problem similar to that of the technique disclosed in Non-Patent Document 2.    Non-Patent Document 1: “Introducing The Passive Anode Surface Emission Cathode”, D. Cooke et al., AIAA 2002-4049    Non-Patent Document 2: “High Voltage Frame and Differential Charging Observed on a Geosynchronous Spacecraft”, B. K. Dichter et al., 6th Spacecraft Charging Technology Conference, 1998    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. H7-52900